1. Field of the Invention
The present invention relates to a microphone that can be formed without a diaphragm by using high-frequency discharge for electric acoustic conversion.
2. Description of the Related Art
Generally, electro-acoustic transducers such as microphones and speakers have a diaphragm. In a microphone, the diaphragm vibrates upon receiving a sound wave and the vibration is detected as an electromagnetic, a capacity, or an optical change to be converted into an electrical signal. In a general speaker, an acoustic signal is electromagnetically converted into a vibration of the diaphragm to be output as a sound wave. Thus, the diaphragm in electro-acoustic transducers is used to convert air vibration into an electric signal and vice versa. In other words, an acoustic system, a machine vibration system, and an electric circuit system of an electro-acoustic transducer are connected via a single diaphragm.
Designing of a microphone begins by setting a control method for the machine vibration system including the directivity of the microphone. Based oh the control method, a resonance frequency of the diaphragm is set and the acoustic circuit system and the electrical circuit system, are designed. Materials and the shape of the microphone are selected and designed to be most suitable for the control method. The control method for the machine vibration system includes mass control, resistance control, and elasticity control. The resonance frequency of the diaphragm is set to be at around the lower limit, the middle, and the higher limit of a main frequency band. A conventional general electro-acoustic transducer, especially a microphone, using any of the methods has a diaphragm. The diaphragm provided therein inevitably limits the frequency response. More specifically, even a diaphragm with the lowest mass provides inertial force to limit frequencies in which the sound can be collected.
In view of the above, an electro-acoustic transducer without a diaphragm is under study. For example, a microphone without a diaphragm is known that uses a laser to detect change in density of air due to a sound wave to convert the sound wave into an electrical signal. Various methods for detecting an acoustic pressure have been studied. Upon collecting sound, a velocity component of a sound wave should be detected together with the acoustic pressure. Among currently available microphones of various methods, a bidirectional microphone can detect the velocity component. Unfortunately, the bidirectional microphone also has a diaphragm, which limits frequencies at which the sound can be collected.
A method is available in which a hot-wire anemometer, which is formed with a semiconductor manufacturing technique, is used to detect the particle velocity of sound waves in the audible frequency. Here, because a degree to which the hot wire is cooled differs according to the particle velocity, the cooling degree can be detected as a resistance change. Unfortunately, as is apparent from the fact that a carbon microphone performs detection in a similar manner, a wide dynamic range is difficult to be obtained by this method.
As an example of a method to provide the electro-acoustic transducer without a diaphragm, Japanese Patent Application Publication No. S55-140400 discloses a method of detecting the particle velocity by using electrical discharge to perform electro-acoustic conversion. The invention disclosed in Japanese Patent Application Publication No. S55-140400 includes: a needle-like discharge electrode; an opposite electrode that surrounds the needle-like discharge electrode with a certain space therebetween. The opposite electrode is made of a conductive material and has a shape of a sphere having a hole through which a sound wave passes. The discharge electrode extends inside the opposite electrode having a sphere shape to roughly the center thereof. From a high-frequency voltage generating circuit, a high-frequency voltage signal, demodulated by a low frequency signal to be converted to a sound wave, is applied to the discharge electrode. Then, a corona discharge corresponding to the high-frequency voltage signal is produced between the discharge electrode and the opposite electrode so that the low frequency signal, i.e., the sound wave is radiated.